The bug can be observed like this:

./phpunit src/Symfony/Bundle/TwigBundle/
#!/usr/bin/env php
PHPUnit 6.5.14 by Sebastian Bergmann and contributors.

Testing src/Symfony/Bundle/TwigBundle/
.......................................................           55 / 55 (100%)

Time: 733 ms, Memory: 26.00MB

OK (55 tests, 190 assertions)

Remaining direct deprecation notices (2)

  2x: The "filter" tag is deprecated since Twig 2.9, use the "apply" tag instead.
    1x in CacheWarmingTest::testCacheIsProperlyWarmedWhenTemplatingIsAvailable from Symfony\Bundle\TwigBundle\Tests\Functional
    1x in CacheWarmingTest::testCacheIsProperlyWarmedWhenTemplatingIsDisabled from Symfony\Bundle\TwigBundle\Tests\Functional

This deprecation is breaking our test suite on 3.4 https://travis-ci.org/symfony/symfony/jobs/525267589#L5408 (not master because the test is marked a legacy there). Bumping the twig requirement to 2.9 is not possible on lower branches (even for master, twig 2.9 is not released yet).
We need to use the filter tag when the apply one is not available

I contributed it on that branch because I did not observe the deprecation on 3.4, but I must have forgotten to update my vendors. The fix will be harder on 3.4 indeed: is there a way to detect if a tag is available from Twig?

There is no AFAIK :/
I thought about registering an internal twig function on 3.4 which would check the existence of a twig 2.9 class e.g. ApplyTokenParser, that could work. But @xabbuh told me that he will be able to look into this bug tonight so I suggest to wait here :)

In case he does find a solution, what should we do with my PR? Close it and let his solution apply 🥁 ? Or is it best to bump the dependency?

I think we would better not bump it (in case Twig 2.9 is not ready in time for the Symfony 4.3 releases) but I don't know tbh :) Let's keep this open until we have more inputs

You can bump the minimum version. I will release the new versions of Twig 1.x and 2.x in the next few days.

On 3.4 :)

cc @xabbuh

I added a warning above, cause I think this should not be merged yet, right?

⚠ Note: don't merge until Twig 2.9 and 1.40 are released, or this will block symfony releases ⚠

Thank you @greg0ire.